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How to Upgrade to a Smart Thermostat with Four Wires | This Old House: Live

Ask This Old House home technology expert Ross Trethewey demonstrates how to install a new smart thermostat without an existing common wire. This video is part of This Old House: Live—an initiative providing daily programming and opportunities to connect with our experts during these challenging times.

Steps for Installing a Smart Thermostat:

  1. Shut the power off and disconnect the old thermostat using the small flathead screwdriver. Ross suggests taking a picture of the wiring configuration to keep track of which wire goes where.
  2. Remove the wall screws holding up the thermostat before disconnecting the wiring to prevent the wires from falling into the wall.
  3. Mount the baseplate of the smart thermostat to the wall using the provided screws.
  4. Wire the smart thermostat to the correct terminals.

a. In general, the heating wire (often white), goes into the “W” port, the cooling wire (often yellow), goes into the “Y” port, the wire that sends power from the HVAC system to the thermostat (often red), goes into the “R” port, the fan wire (often green), goes into the “G” port, and the common wire (often blue), which should go into the “C” port.

b. If you don’t have a common wire, Ross recommends wiring the G wire into the common port instead. Most people don’t use the “fan only” mode, so while you’d give up control over the fan in the system, you’d gain constant power to the smart thermostat. You’ll also have to change the wire at the furnace from G to C.

5. Connect the thermostat to the baseplate.

6. Configure the thermostat following the manufacturer’s settings.


Ross installed a Google Nest Thermostat E. The kit comes with the baseplate, thermostat, and required screws to secure it to the wall.